In a hybrid powertrain, an electric energy conversion device, for example a motor/generator, may be used to absorb and/or supply torque to improve powertrain efficiency and fuel economy performance. In one example, a hybrid powertrain may include an internal combustion engine and an electric motor connected in series to a torque converter. In this configuration, the efficiency of the hybrid powertrain may be improved by engaging the lockup clutch of the torque converter and operating the internal combustion engine at a reduced load and using the electric motor to provide additional torque to meet driver demand. One approach that employs this configuration is described in U.S. Pat. No. 6,835,160.
The inventors herein have recognized that the above approach may have some issues. In particular, during some operating conditions, the lockup clutch may be unable to lock the torque converter. For example, desired early torque converter lockup may result at a condition when a considerable torque differential may exist across the torque converter greater than that of the capacity of the lockup clutch. Accordingly, it may be beneficial to reduce the torque differential across the torque converter in order to engage the lockup clutch. However, in the above described configuration, since both the internal combustion engine and the electric motor are positioned upstream of the torque converter, torque adjustment may only be applied to one side of the torque converter and the torque differential may not be reduced under some conditions. Thus, attempting to lockup the torque converter may cause deteriorated driving conditions and component degradation due to the rough or unsuccessful engagement of the lockup clutch.
The above issues may be addressed by, in one example, a hybrid vehicle propulsion system comprising: an internal combustion engine; a torque converter including a lockup clutch the torque converter having an input and an output, the input coupled to the internal combustion engine; an electric energy conversion device coupled downstream of the torque converter output; an electric energy conversion device coupled upstream of the torque converter input; and a control system for adjusting torque output of the hybrid propulsion system, the control system adjusting the torque output of the electric energy conversion device during a torque converter lockup clutch engagement transition event.
Thus, by adjusting the torque output of the electric energy conversion device downstream of the torque converter, torque may be provided to final drive/wheels to meet driver demand while also reducing torque load at the output of the torque converter. The reduced torque load at the output of the torque converter may, in turn, initiate reduction in torque differential across the torque converter enabling smoother and/or earlier torque converter lockup, even when lock-up clutch capacity may be insufficient. Further, by positioning torque sources upstream and downstream of the torque converter, torque load may be adjusted to both the input and the output of the torque converter so that reduced torque differential may be achieved. In this way, efficiency and fuel economy of a hybrid powertrain may be improved while reducing deteriorated driving conditions and meeting driver demand.
Further, in another example, the transmission shifting and the torque converter lockup state may be adjusted based on the distribution of engine power between multiple propulsion paths in order to improve overall hybrid powertrain efficiency. In certain operating conditions, a portion of the engine power may be distributed to one propulsion path to power the electric motor (downstream of the transmission) and/or charge the battery while the remaining portion of the engine power may be transferred through the transmission in another propulsion path to provide power to the drive wheels. In these conditions, the transmission shifting and torque converter lockup scheduling may be adjusted to compensate for the reduction in net input power to the transmission. In this way, hybrid powertrain efficiency and fuel efficient may be improved while meeting the driver demand.
In still another example, a control architecture for a hybrid propulsion system may be provided that considers the tractive effort capabilities of the respective hybrid powertrain torque sources for a selected operating condition and adjusts the transmission shifting and the torque converter lockup state in order to distribute power flow accordingly. In particular, the control architecture may adjust the transmission shifting and torque converter lockup state based on the tractive effort capabilities of the electric torque sources, including the battery state of charge. In this way, hybrid powertrain efficiency and fuel efficiency may be improved while meeting driver demand.